Images on electronic displays are derived from an array of small picture elements known as pixels. In color displays, these pixels comprise three color elements that produce primary colors red, blue and green, typically. Usually arranged as squares or rectangles, these pixels can be characterized by pixel pitch, P, a quantity that measures the density of pixels per unit distance. A typical cathode-ray tube has a pixel pitch of 0.3 mm. Typical small computer screens have a width:height ratio of 4:3. Two common arrangements for pixel arrays for computer displays are the 640.times.480 and the 1024.times.768 configuration.
Large displays using LCD technology are recognized as being difficult to manufacture. It has been suggested that large displays can be manufactured by assembling a plurality of display tiles. Such tiles are characterized by visually disturbing seams resulting from gaps between adjacent pixels on the same and/or adjacent tiles. Thus, the image portrayed by using a seamed panel appears segmented and disjointed. Therefore, it is desirable to fabricate a tiled, flat-panel display which does not have noticeable or perceptible seams.
The pixel pitch in electronic displays is set so that the minimum viewing distance will produce an imperceptible seam. With a pixel pitch P=0.3 mm, the minimum viewing distance is on the order of one meter. The minimum viewing distance will increase with the pixel pitch; therefore, when designing for the purpose of visually eliminating the seams, there is very little latitude in selecting the pixel pitch.
Flat-panel displays include liquid crystal displays (LCDs), active matrix LCDs (AMLCDs), plasma displays (PDs), field emission displays (FEDs), electroluminescent displays (ELDs) and digital mirror displays (DMDs), all of which depend on the microfabrication of the key components carrying the pixel patterns. AMLCD is a technology currently favored by the industry. For purposes of clarity, the term "LCD" is used herein, but is intended to describe all flat-panel displays.
From a practical point of view, the microfabrication yield is unacceptable for large displays, due to the unacceptable number of manufacturing rejections. The inventors, therefore, have determined that small pixel arrays (tiles) can be microfabricated and, after appropriate selection, assembled together to form a larger display configuration. However, past attempts to accomplish this have led to visible seams, due, in large part, to the dimensions required by tile assembling, which goes beyond even the pixel spacing required of monolithic displays. This is essentially why few attempts have been made to achieve large, color, "seamless", tiled panels.
In co-pending U.S. patent applications Ser. Nos. 08/593,759 and 08/571,208, which were filed on Jan. 29, 1996, and Dec. 12, 1995, respectively, a method of constructing a seamless, tiled, flat-panel display is illustrated. The teachings of these companion applications are meant to be incorporated herein by way of reference.
The present invention provides unique hardware and a method for achieving luminance correction in a "seamless", tiled display, comprising a tiled mosaic of individual LCDs. In a commercially acceptable tiled display, the color and brightness have to be uniform for each tile, i.e., over the entire range of input video signals to be rendered, there should be no apparent differences in brightness or color between tiles.
There are several sources of inter-tile color differences, including differences in the color coordinates between tiles, threshold and transmission voltages at the seams, etc.
Moreover, the optical performance of the display can be characterized by parameters that describe the voltage input to picture elements (pixels) and the resulting transmission of the elements. For example, AMLCDs have threshold voltages V.sub.TH and V.sub.DMUX for maximum and minimum transmission, T.sub.max and T.sub.min. The pixel optical gain, V.sub.SL, can be described as the slope of the transmission-voltage curve. Color coordinates may also vary. A similar set of parameters can be identified for other types of flat-panel displays. In the extension to tiled displays, additional parameters related to the quality of the display near the edge can be identified, for example, due to the filling of the liquid crystal material. Other optical components of the display may also vary.
The method of this invention takes into account the condition in a tiled display where the color coordinates between tiles are approximately constant, but the transmission of light through the pixel assemblies slowly varies across a tile for each primary color and, additionally, differs from tile to tile. In particular, with this invention, the inventors seek to correct discontinuities in primary-color luminance at border regions.
The inventors have also developed a method of characterizing spatial pixel transmission variations. After characterizing these variations, the invention determines the correction to be applied and compresses the data so that it can be used by a unique hardware arrangement for implementing the required video correction to the tiled display.
It is an object of this invention to provide an improved flat-panel, tiled display.
It is another object of this invention to provide a flat-panel, tiled display that is seamless and constructed with matched, color-coordinated tiles.
It is a further object of the invention to provide both a luminance-correcting circuit that uses a reduced amount of correction data, and a real-time correction method for a tiled, LCD display.